One example of gas sensor based on measuring the spectrum absorptions of emitted light by a gas is described in US 2008/0283753, wherein the pass band of a first filter is arranged within the pass band of a second filter and the evaluating device forms the difference of the signals and normalizes it to the signal.
Using such a sensor in relatively harsh environments such as exhaust systems in ships, vehicles etc. will however expose the delicate parts environment within the exhaust stack that may comprise a wide range of particles and gasses that could damage them, or just reduce their lifetime. One option would be to protect the parts with sight glasses such that they becomes isolated from the harsh environment, but the transparency of these may then be reduced over time by settlement of particles etc.
Another example of a gas sensor is to be found in EP 2 604 999 disclosing a gas analyser for the absorption-spectroscopic in-situ determination of at least one chemical and/or physical parameter of a gaseous measurement medium, wherein the gas analyser includes a first housing; at least one laser as a radiation source, which laser is arranged in the first housing; at least one first process window for coupling the radiation emitted by the laser into a measurement medium; and at least one detector by which, following interaction with the measurement medium, the radiation is detected. The sensor includes openings in the side of the probe the gas thus passing directly from the outside of the probe transversing the measuring area of the probe directly through these openings under the outside flow conditions such as flow rates of the gasses to that of the inside of the probe. This has some disadvantages in that there is no control of the flows and conditions within the measuring region of the probe such as the exchange rate of gasses.